This application generally relates to the field of geophysical prospecting. More specifically, the application relates to the field of streamer design. An apparatus that includes a solid-core streamer with particle motion sensors in the core is disclosed. Methods of operating the apparatus are also disclosed.
In the oil and gas exploration industry, marine geophysical prospecting is commonly used in the search for hydrocarbon-bearing subterranean formations. Marine geophysical prospecting techniques may yield knowledge of the subsurface structure of the Earth, which is useful for finding and extracting hydrocarbon deposits such as oil and natural gas. Seismic and electromagnetic surveying are two of the well-known techniques of geophysical prospecting.
For example, in a seismic survey conducted in a marine environment (which may include saltwater, freshwater, and/or brackish water environments), one or more seismic energy sources are typically configured to be submerged and towed by a vessel. The vessel is typically also configured to tow one or more (typically a plurality of) laterally spaced streamers through the water.
Some techniques of geophysical prospecting involve the simultaneous use of seismic and electromagnetic survey equipment. For example, during such a survey, equipment that includes streamers with electromagnetic sensors may be similarly towed behind a vessel. Electromagnetic surveying includes imparting an electric field or a magnetic field into the Earth's subterranean formations and measuring components of the resultant electromagnetic fields. Data collected during such a marine geophysical survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located.
A seismic survey commonly employs seismic energy sources such as an array of air guns that produce a seismic wavefield when activated. As used herein, a “wavefield” is a component of seismic data which can be represented by a single velocity field with vertical and lateral variations. In a marine seismic survey, the wavefield typically travels downward through a body of water overlying the subsurface of the earth. Upon propagating into the Earth, the seismic wavefield is then at least partially reflected by subsurface reflectors. Such reflectors are typically those interfaces between subterranean formations having different elastic properties such as density and sound wave velocity, which may lead to differences in acoustic impedance at the interfaces. The reflected seismic wavefield is detected by the sensors such as particle motion sensors and/or pressure sensors in the seismic streamers. A record is made in the recording system of the signals detected by each sensor (or by groups/networks of such sensors). The recorded signals are thereafter interpreted to infer the structure and composition of the subterranean formation.
A typical streamer may be quite long, typically multiple kilometers in length. Some geophysical surveys may be conducted with a single streamer, while some surveys use multiple streamer systems including one or more arrays of streamers. The individual streamers in such arrays are generally affected by the same types of forces that affect a single streamer.
One of the most common types of pressure sensor used in marine geophysical surveying is a hydrophone. A hydrophone is generally understood to be an omnidirectional device. Such hydrophones therefore generally cannot distinguish between the directions of the up-going and down-going wavefields. In particular, seismic energy directly reaching the hydrophones from the source cannot be easily distinguished from the various reflections from the surface of the water and the seafloor (e.g., the “source ghost” and the “receiver ghost”). As a consequence, wavefields of both the source and receiver ghost may interfere with primary reflections, which contain the desired information about the subterranean formations, reducing seismic image resolution and reducing the usefulness of seismic data for reservoir delineation and characterization.
Due to the omnidirectional reception nature of hydrophones, particle motion sensors have also been employed to detect vector quantities during marine seismic survey operations. The term “particle motion sensor” should be understood in the context of this disclosure as referring to any of various types of sensors, including velocity meters, accelerometers, geophones, pressure gradient sensors, particle displacement sensors, etc.
In a multi-component streamer that includes both pressure sensors and particle motion sensors, the combination of signals from pressure and particle motion sensors may be used to remove the “ghosting” effects during seismic data processing. One example of this is described in U.S. Pat. No. 7,684,281, which is incorporated herein by reference in its entirety.
The design of a multi-component streamer thus may take into account multiple factors, including the different responses and noise characteristics of pressure sensors and particle motion sensors, and also electromagnetic sensors. It is desirable for a streamer to be durable and less prone to mechanical and other sources of noise. Improvements may also be desirable in areas such as signal-to-noise ratio (e.g., for both pressure sensors and particle motion sensors), diameter (e.g., a reduced diameter may give reduced drag and easier handling), robustness, etc.